The present invention relates to a process for the production of ethylene polymers by the polymerization of ethylene or the copolymerization of ethylene with one or more olefins using a chromium-based catalyst deposited on silica. It relates more particularly to a process for producing ethylene polymers having an improved compromise between stiffness, processability and resistance to slow crack growth.
It is well known to produce ethylene polymers (homopolymers and copolymers) by the use, in ethylene (co)polymerization of chromium-based catalysts deposited on a silica-based support. It is also known that the activity of such catalysts can be improved by subjecting them to a heat treatment in air at high temperature. These activated catalysts are very suitable for producing ethylene polymers which combine low melt flow indices (that is to say high average molecular masses) with high densities. Ethylene polymers having a high density generally have a high stiffness, thereby making it possible to reduce the weight of articles fashioned therefrom without a significant loss of mechanical properties. However, the increase in stiffness is generally to the detriment of the resistance to slow crack growth. For many applications of ethylene polymers, such as hollow bodies and pipes intended to contain or transport various liquids and fluids, it is paramount to have available polymers which exhibit both a high stiffness and a high resistance to slow crack growth.
Patent Application EP-A-882,740 describes means for improving the crack resistance of ethylene polymers obtained using chromium- and titanium-based catalysts deposited on supports comprising silica. This result is achieved by the use, for ethylene (co)polymerization, of a catalyst produced as follows: chromium deposition on the support, dehydration of a chromium-based catalyst by heating in an atmosphere of dry inert gas at a temperature of at least 300xc2x0 C., titanation of the chromium-based catalyst by bringing it into contact with a titanium compound at a temperature of 300xc2x0 C. at least in an atmosphere of dry inert gas and, finally, activation of the titanated chromium-based catalyst by heat treatment in dry air at a temperature of 500 to 900xc2x0 C. The purpose of the step of dehydrating the chromium-based catalyst is to prevent the formation of TiO2 by the reaction of water with the titanium compound during the subsequent titanation step.
The object of the present invention is to provide a process for the production of ethylene polymers using a chromium-based catalyst deposited on a support essentially consisting of silica, which exhibit a good compromise between processability, rigidity and resistance to slow crack growth.
For this purpose, the invention relates to a process for the production of ethylene polymers by the polymerization of ethylene or the copolymerization of ethylene with one or more C3 to C8 olefins in contact with a chromium-based catalyst deposited on a support essentially consisting of silica, in which process the chromium-based catalyst deposited on the silica is subjected in succession to a heat treatment in nitrogen at a temperature greater than 350xc2x0 C. and less than 850xc2x0 C. and then to a heat treatment in air at a temperature greater than 350xc2x0 C. and less than 800xc2x0 C. before it is brought into contact with the ethylene and, optionally, one or more olefins under polymerizing conditions for producing an ethylene polymer.
The invention is based on the surprising observation that the heat treatment in nitrogen, under the thermal conditions defined above, prior to the conventional thermal activation in air of a chromium-based catalyst deposited on a support essentially consisting of silica, contributes to improving the compromise between stiffness, processability and resistance to slow crack growth of the ethylene polymers produced. The process according to the invention consequently allows, in particular, the production of ethylene polymers having a markedly improved resistance to slow crack growth for a given stiffness and given processability. As a corollary, it allows the production of ethylene polymers with a higher density, and therefore a higher stiffness, without an appreciable reduction in the resistance to slow crack growth.
The heat treatments, in nitrogen and in air respectively, are preferably carried out at a temperature which is at least 480xc2x0 C. and, moreover, does not exceed 760xc2x0 C. Excellent results are obtained when the heat treatments, in nitrogen and in air respectively, are carried out at a temperature which is at least 530xc2x0 C. and does not exceed 715xc2x0 C. It is understood that the heat treatment in nitrogen and the heat treatment in air must not be carried out at the same temperature. Preferably, the temperature of the heat treatment in air does not exceed that of the heat treatment in nitrogen. Advantageously, similar, but not identical, temperatures are used for the two successive heat treatments.
The duration of the heat treatment in nitrogen is not critical. In general, it will be between a few minutes and a few hours, usually between 10 minutes and 20 hours. Excellent results are obtained with durations of heat treatment in nitrogen ranging from 6 to 16 hours. Likewise, the duration of the heat treatment in air is not critical. Usually, it will between 5 minutes and 6 hours. Excellent results are obtained with durations of heat treatment in air ranging from 30 minutes to 2 hours.
The heat treatment in nitrogen and in air of the chromium-based catalyst supported on silica may be carried out using any known method for bringing gases and solids into contact with each other, such as in a static bed or a fluidized bed. Advantageously, these heat treatments take place over a fluidized bed, the supported catalyst being kept in the fluidized state by means of the gases, nitrogen and air respectively, used in the two heat treatment steps.
The rise in temperature, up to the temperature of the heat treatment in nitrogen, is advantageously accompanied by flushing by means of a non-oxidizing gas such as, for example, nitrogen, carbon monoxide, hydrogen and mixtures thereof. Preferably, it is accompanied while flushing with nitrogen.
At the end of the heat treatment in air, the catalyst is, in a known manner, cooled down to room temperature for the purpose of recovering it and storing it under the protection of nitrogen until it is used for the polymerization. To do this, the air may be replaced with nitrogen at the end of the heat treatment in air and the temperature of the catalyst may be gradually reduced down to room temperature while flushing with nitrogen. A preferred method of implementation consists in cooling the catalyst gradually in air, generally down to a temperature of between 300 and 400xc2x0 C., advantageously down to approximately 350xc2x0 C., and then continuing the cooling down to room temperature, keeping the catalysts flushed with nitrogen before recovering and storing it under a protection of nitrogen.
The support for the chromium-based catalyst used in the process of the invention essentially consists of silica, that is to say it is substantially free of other oxides. The silica content of these supports is generally at least 99.9% by weight.
In general, the silica support has a specific surface area ranging from 100 to 800 m2/g, measured according to the BET volumetric method in British Standard BS 4359/1 (1984). Usually, the specific surface area does not exceed 500 m2/g. Preferably, it ranges from 150 to 400 m2/g.
Moreover, the silica support generally has a pore volume of approximately 0.5 to 4 cm3/g. Preferably, the pore volume is approximately 1 to 3 cm3/g. The term xe2x80x9cpore volumexe2x80x9d should be understood to mean the pore volume measured according to the nitrogen penetration method (BET) with reference to British Standard BS 4359/1 (1984).
The chromium-based catalyst deposited on a support essentially consisting of silica used in the process according to the invention usually contains approximately 0.1 to 5% by weight and even more particularly 0.5 to 1.5% by weight of chromium (these weights being expressed with respect to the weight of supported catalyst). Of course, it is possible to use mixtures of chromium-based catalysts deposited on supports essentially consisting of silica.
The way in which the silica-supported chromium catalyst is obtained is not critical. By way of non-limiting examples of chromium-catalysts deposited on a support essentially consisting of silica that can be used in the process of the invention, mention may be made of the commercial catalysts EP30X (sold by Crosfield) and HA 30W (sold by Grace Davison).
According to the invention, the silica-supported chromium catalysts having undergone successive heat treatments in nitrogen and in air, under suitable conditions of temperature and duration, are used for the polymerization of ethylene or for its copolymerization with C3 to C8 olefins. By way of examples of olefins that can be used as comonomers, mention may be made of propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. Of course, several olefins may be used simultaneously.
The process according to the invention applies particularly well to the manufacture of ethylene homopolymers and of copolymers containing at least 90 mol% ethylene, and especially random copolymers and terpolymers of ethylene with 1-butene and/or 1-hexene or even of block copolymers obtained by the sequential polymerization of mixtures of ethylene and of butene and/or hexene.
The (co)polymerization is carried out, in a known manner, by bringing ethylene and, optionally, other olefins into contact with the catalyst under polymerizing conditions known per se. The (co)polymerization of the ethylene may be carried out using any known processxe2x80x94in solution in a solvent, in suspension in a hydrocarbon diluant or in the gas phase. Advantageously, it is carried out in suspension in an inert diluant, such as isobutane. Typically, the polymerization temperature is between 20 and 130xc2x0 C. and the pressure is between atmospheric pressure and 100xc3x97105 Pa, preferably between 10xc3x97105 and 55xc3x97105 Pa.
The ethylene polymers produced according to the process of the present invention exhibit an excellent compromise between stiffness, processability and resistance to slow crack. They may be used in any conventional process for converting thermoplastics, such as, for example, extrusion and extrusion-blow moulding. They are very suitable for the extrusion-blow moulding of hollow bodies and for the extrusion of pipes, in particular for the extrusion-blow moulding of hollow bodies such as hollow bodies of reduced weight (xe2x80x9clightweightingxe2x80x9d).